We propose to continue our studies of the P2-P4 system of E. coli phages, in particular with regard to three topics: (1) The control of capsid (= head) size by P2 and P4 genes which leads proteins encoded by P2 to assemble either as large (= P2 size) heads or, in the presence of P4, as "small" (= P4 size) heads. Plans include continued studies of P2's sir mutations which let P2 assemble large heads even in the presence of P4 (the three already mapped sir mutations all occur in gene N which encodes the major head protein); a search for P2 mutations causing P2 to assemble small heads even in the absence of P4 (hence preventing production of P2 itself but sitll allowing P2 to serve as helper for P4) and the analysis of such mutations, if found; study of P4 mutations, especially in its head size direction gene sid, using P4 mutants isolated by selection procedures based on the P2-P4 head size control interactions; and further studies of these interactions. (2) The role of P4's ash (cI) gene in the negative control of P4 gene expression. Some of the results of our previous studies of ash mutations are difficult to interpret in terms of the ash gene encoding a repressor protein. Proposed are tests to determine the effects of already recognized ash-amber mutations on the control of P4 gene expression; identification of P4 promoters that are 1pf gene, which may be identical to rpoA; identification of the P4 genes requiring the Lpf+ controlled by ash; and a search for a possible countertranscript from the ash region. (3) The nature of E. coli mutatios (1pf) specifically interfering with P4's lytic growth iP2 lysogens. Plans include identification of the function for their expression; and studies of P4 mutants, including certain P4 ash mutants, able to grow in Lpf- hosts.